U.S. Pat. No. 5,436,784, owned by the assignee of the present invention, discloses the use of thermal models to determine when an overload condition has been reached relative to calculated temperatures in the motor. The contents of the '784 patent are hereby incorporated by reference. The thermal models include one model for a start condition of the motor, defined as when the measured rotor current is relatively high, specifically, greater than 2.5 times the full load rated motor current, and a second model for a run condition of the motor, i.e. when the motor is at rated speed, defined as when the rotor current is less than 2.5 times the maximum rated current.
The thermal models are determined using values of full-load motor current, locked rotor current, and locked rotor thermal time, which are available from the manufacturer. Threshold values of temperature are established for both conditions (both thermal models), again using the manufacturer's information. The thermal models include a representation of the heating effect in the rotor, a representation of the thermal capacity of the rotor (the product of the specific heat of the body being heated times the mass of the body), and a cooling effect present in the rotor, i.e. the ability of the motor to give off heat during operation.
FIG. 1 shows a known electrical analog of the thermal model approach for a “start” condition of the motor, while FIG. 2 shows a known electrical analog for a “run” condition. In the figures, R1 refers to the locked rotor electrical resistance (in ohms), R0 refers to the running motor electrical resistance, IL refers to the locked rotor current, Ta refers to the locked rotor time with the motor initially at ambient temperature, T0 refers to the locked rotor time with the motor initially at an operating temperature, while TD refers to a time dial number needed to reach the trip temperature, and SF refers to a “service factor” value, i.e. a threshold value.
In the thermal model arrangement disclosed in the '784 patent, the run condition time-current curve did not match the time-current curve of the start condition, resulting in less than optimum protection for the motor, due to either the rotor or the starter being individually the limiting factor in the protection, instead of both having equal effect. The curves can be matched, i.e. made continuous, by a time constant which relates starter condition to the rotor condition, but often, the time constant, although usually known to the manufacturer, is not made available to the motor customers.
In such cases, a time constant must be determined and then added to the thermal protection calculation, but it may not be accurate, which results in less than optimum protection performance. One example is cyclical loads to the motor, where the temperature in the motor may regularly rise above a conventional threshold temperature but the cyclical nature of the load does not result in a true temperature overload appropriate for a tripping action. With an incorrect or less than optimum time constant, a trip may occur which is unnecessary.
In this invention, a time constant is calculated which in fact does remove the discontinuity between the time-current curves of the start condition and run condition thermal models, resulting in a continuous time-current curve which in turn results in improved protection, decreasing unwarranted tripping actions.